The invention relates generally to optical waveguides, and relates more particularly to waveguide photodetectors for converting optical signals into electrical signals.
Optical interconnects can offer significant advantages over electrical circuitry in the field of advanced microelectronics. One possible implementation of a deeply-scaled optical interconnect system is based on silicon-on-insulator (SOI) technology, in which optical waveguides are formed (e.g., in accordance with CMOS technology) on a thin silicon device layer. Such a system requires four major components: a light source, an optical modulator for encoding data into light pulses, a waveguide for efficiently transporting light across the chip and a photodetector for converting optical signals into electrical signals. The lack of an effective photodetector for such applications has, however, impeded the development of highly scalable SOI interconnects to date.
In particular, the photodetector must be made from silicon (Si) or germanium (Ge) in order to be compatible with CMOS processing. However, if the waveguides of the interconnect system are made of Si, the operational wavelength range for light propagation is restricted to approximately 1.1 to 1.8 microns, where Si is transparent. Thus, a Ge photodetector would seem preferable. Typical Ge detectors, however, are vertically incident and thus are not compatible with monolithically integrated photonics on a SOI substrate. Horizontal in-plane implementation is also made difficult due to the large differences in relative refractive index between Si (approximately 3.5) and Ge (approximately 4.2), which complicates efficient coupling of light from the waveguide to the photodetector. Typical Ge detectors are also multimoded, which prevents efficient coupling between the waveguide and the photodetector. Moreover, typical Ge detectors in general suffer from large dark current and impaired bandwidth.
Thus, there is a need for a scalable waveguide photodetector that efficiently couples light from a waveguiding region to a detection region.